This invention relates generally to radio frequency systems for identifying passive resonant circuits.
Passive resonant circuits have been placed on goods, excited with swept radio frequency energy and resonant circuits detected to permit identification of the circuits. For example, a plurality of such circuits which respond at different frequencies may be incorporated in labels affixed to goods to provide a code. Such systems may be used in retail establishments to prevent pilferage, at check-out stands to check out goods, and in material handling to route goods such as in warehousing and storage. When such coded labels are placed on goods themselves, they are transported with the goods and permit identification of the goods at various locations as they are shipped in transit, stored or sold.
It is known in the prior art to use coded tags with passive radio frequency resonant circuits in the form of spirals, strips or slots in metal. Principally, two methods have been used for detection of the resonant circuits. One such method employs the principle of the grid dip meter. It is known that when the field of an oscillator is intercepted by a circuit resonating at the oscillator frequency a certain amount of energy is absorbed from the field. This increases the power consumption which in a vacuum tube oscillator shows up as a change in grid current. Multiple resonant circuits have been detected by shifting the oscillator frequency step by step or by sweeping the frequency to cover all potential resonant frequencies of the circuits associated on the coded tag.
Another method which is an extension of the grid dip method uses one transmitter which serves to transmit the radio frequency energy with increasing frequency, either step-wise or swept, and a separate receiver for receiving the energy. The receiver is arranged to detect changes in the intensity of the received swept signal from the transmitter indicating that a passive resonant circuit has absorbed transmitted energy at a particular frequency. Such circuits have also been used which detect the change in phase of the received signal. The signal is generally detected in an environment of very high signal caused by clutter and the direct transmission, and consequently the circuits must be able to detect a weak signal in the presence of high "noise" signals. Each of the above methods is highly limited in range for a given size resonant circuit.
There is need for a radio frequency resonant circuit detection system which can discriminate against clutter or reflected radio frequency energy and against direct transmitted energy to provide signals when an associated resonant circuit is excited.